ABB REG670 - Page 340

of IED A1 must have a delayed operation in order to avoid maloperation. The
sequence of events during the fault can be described using a time axis shown in
Figure 176.
I> I>
A1 B1
Feeder
Time axis
t=0 t=t
2
t=t
3
The fault
occurs
B1 trips and
A1 starts
Breaker at
B1 opens
=IEC05000205=2=en=Original.vsd
Fault
t=t
4
Protection
A1 resets
t=t
1
B1 and A1
start
IEC05000205 V2 EN
Figure 176: Sequence of events during fault
where:
t=0 is when the fault occurs
t=t
1
is when protection IED B1 and protection IED A1 start
t=t
2
is when the trip signal from the overcurrent protection at IED B1 is sent to the circuit breaker.
t=t
3
is when the circuit breaker at IED B1 opens. The circuit breaker opening time is t
3
- t
2
t=t
4
is when the overcurrent protection at IED A1 resets. The protection resetting time is t
4
- t
3
.
To ensure that the overcurrent protection at IED A1 is selective to the overcurrent
protection at IED B1, the minimum time difference must be larger than the time t
3
.
There are uncertainties in the values of protection operation time, breaker opening
time and protection resetting time. Therefore a safety margin has to be included. With
normal values the needed time difference can be calculated according to
Equation 186.
Section 9 1MRK 502 071-UEN -
Current protection
334 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual